The invention relates to the stationary and rotating blades of a low-pressure steam turbine, and in particular to a coating for such blades used to increase the efficiency of the low-pressure steam turbine.
The expansion of the turbine steam in a low-pressure turbine to condenser pressure usually results in a range for wet steam. The mass content of the condensation water in the wet waste steam can be up to 14%. The impulse of the entire mass flow-rate of the turbine steam is preserved, independently from the condensation water content. However, the presence of a liquid phase in the rotating and stationary elements of the turbines results in increased dissipative losses. In a low-pressure turbine, about 12-14% of the mass flow-rate is generated in the form of water. This moisture loss results in a loss of efficiency of the low-pressure turbine of approximately 6-7%, which corresponds to a loss of efficiency of approximately 1-2% of an entire steam power plant. In combination and nuclear power plants, the contribution of power of the low-pressure turbine in relation to the overall plant power is slightly higher by comparison, so that the loss in overall efficiency due to moisture losses is approximately 2-2.3% or 3-3.5% overall.
The extent of losses depends for the most part on the size of the water droplets. In most cases, only small droplets, in the range of micrometers, are contained in the steam phase. The water droplets maintain their size and do not coalesce into larger droplets as long as they keep floating or flowing in the steam. Similar to a vapor, they flow along with the steam path that exerts the impulse onto the turbine blades. As long as the droplets remain in this small size range, they do not have an adverse effect on either the operation or on the performance of the turbine. However, as they flow through the stationary and rotating blades, the droplets grow. During the contact with metal surfaces, probably in particular with the concave metal surfaces of the stationary blades, the small condensate droplets spread on the surface and form a closed condensate film that flows on the blades over the concave or convex surfaces subject to the effect of the shearing forces of the steam. At the trailing edge of the blade, the fluid film leaves the surface and is accelerated and divided by the rotating blades. The droplets generated by this division have a larger diameter than the droplets created by spontaneous condensation.
By centrifugal forces, these larger droplets are spun outward by the rotating blades in the direction towards the turbine housing. This means that a part of the impulse of the working medium is not transferred onto the blades, which results in a moisture loss that reduces the degree of efficiency of the low-pressure turbine. This phenomenon is even stronger the more that the size and mass of the droplets, and therefore of the centrifugal force, increase. Furthermore, accumulations of water at the inside surfaces of the housing of the low-pressure turbine result in dissipative friction losses on the rotating blade tips and blade shrouds. Finally, enlarged droplets with diameters in the range from 100-200 xcexcm and speeds in the range of more than 250 m/s cause droplet impingement erosion due to the impact of the droplets. This droplet impingement erosion depends greatly on the specific materials being used in the turbine blades. It has been found that blade materials of titanium and titanium alloys, which are used preferably for the large blades of the low-pressure turbine, are especially susceptible.
For a long time, attempts have been made to provide the blades of turbines with a coating that would increase the droplet impingement erosion resistance of the blades, thus extending their useful life. DE 37 24 626 describes a coating for the blades of a steam turbine that consists of a hard, wear-resistant ceramic material or of a multi-layer coating of active metal and a shroud layer of a ceramic material. The coating made from ceramic material is used to increase the droplet impingement erosion resistance of the blades, while the layer of active material improves the adhesion of the coating.